The present invention relates to electronic musical instrument keyboard apparatus equipped with mass body (or hammer) units.
Keyboard apparatus of electronic musical instruments have been known from, for example, Japanese Patent No. 3,060,930, in which keys and mass body units corresponding to the keys are supported on a frame and each of the mass body units is pivotable in response to operation of the corresponding key.
FIGS. 8A and 8B are views, with parts taken away, showing an inner construction and mass body units of a conventionally-known keyboard apparatus of the above-mentioned type. In FIG. 8A, a plurality of white and black keys 51 and 52 are disposed in parallel to one another, and each of the white and black keys 51 and 52 has at its rear end portion a pivot point portion 51a or 52a supported by a key support section 53a of the frame.
Each of the white keys 51n has a force transmitting portion 51b projecting downward from its front lower surface, and the force transmitting portion 51b has a distal-end engaging portion. Resilient member 54 is secured to the lower surface of the engaging portion. When depressed, the white key 51 is guided vertically by a key guide 53c projecting upward from a front horizontal surface portion 53b of the frame.
Although not shown in the figures, each of the black keys 52 has a force transmitting portion projecting downward from its front lower surface portion and then bending forward and having its distal-end engaging portion engaging with the corresponding mass body unit, and a resilient member is secured to the lower surface of the engaging portion. When depressed, the black key 52 is guided vertically along a key guide projecting from a middle horizontal surface portion 53d; the key guide of the black key 52 is identical in construction to the key guide 53e of a black key shown as removed in the figure.
Reference numeral 55 indicates the mass body unit, and the mass body units 55 of generally the same construction are provided below and in one-to-one corresponding relation to the white and black keys 51 and 52. Leaf spring 56 is disposed through a window 53f between each of the white key 51 and the corresponding mass body unit 55; more specifically, it is connected at its one end to the interior of the white key 51, passed through the window 53f formed through the thickness of the frame and connected at the other end to the mass body unit 55. Elongated slanting plate 53g, extending from a lower front end position of the frame 53 rearward obliquely upward as viewed in a front-rear direction of the keyboard apparatus, lies along a length over which the keys 51 and 52 are arranged (key-arranged direction). Mass body unit support section 53h in the form of a substantial cylindrical column is provided on and along the upper end of the elongated slanting plate 53g, and each of the mass body units 55 is pivotably supported on the support section 53h. Printed circuit board 57 is provided under the slanting plate 53g, and through-holes 53i are formed in the slanting plate 53g. Two rows of key switches 58, two per through-hole 53i, are provided on the printed circuit board 57.
As shown in FIG. 8B, the mass body unit 55 includes a resin-made base section 3 and an elongated member 59. The base section 3 has a pivot point portion 3a of a semicircular sectional shape kept in fitting engagement with the mass body unit support section 53h of the frame 53. The pivot point portion 3a has a projection 3b engaged in a groove formed in the mass body unit support section 53h. 
The pivot point portion 3a is bifurcated at its front end into main and auxiliary driven portions 3c and 3d, and these main and auxiliary driven portions 3c and 3d engage with the engaging portion, provided at the distal end of the force transmitting portion 51b of the white key 51, via the resilient member 54. Each of the black keys 52 engages with the corresponding mass body unit 55 in a similar manner to the white key 51.
Downwardly-projecting switch driving portion 3e is provided below and between the auxiliary driven portion 3d and the pivot point portion 3a, and this switch driving portion 3e sequentially depresses the two key switches 58 with a given time difference as a front upper surface area of any one of the white and black keys 51 and 52 is depressed.
The base section 3 and the elongated member 59 are interconnected integrally via a connecting portion 3f located below and rearwardly of the pivot point portion 3a. The spring 56 has an engaging portion 3g located on the outer periphery of the pivot point portion 3a. The elongated member 59 is in the form of a metal rod of a circular cross-sectional shape extending along the length of the corresponding key and produces a great moment of inertia when it pivots. The elongated member 59 has, at its rear end remotest from the pivot point portion 3a, a bent extension portion 59a on which its mass concentrates. As a human player depresses one of the white keys 51 with a finger in the example of FIG. 8A, the corresponding mass body unit 55 pivots, a reactive force corresponding to a moment of inertia of the elongated member 59 is given from the white key 51 to the player's finger. Then, once the human player releases the finger from the depressed white key 51, the mass body 55 pivots back to the original position.
Elongated lower-limit stopper (lower movement limiting member) 60 is disposed along the key-arranged direction beneath a rear end portion of the frame 53, while an elongated upper-limit stopper (upper movement limiting member) 61 is disposed along the key-arranged direction above the rear end of the frame 53. Generally, each of these elongated stoppers 60 and 61 is in the form of a felt belt and defines a lower- or upper-limit position in a pivoting range of the mass body unit 55 by a rear end lower surface of the elongated member 55 or upper surface of the bent extension portion 59a abutting against the felt belt. The black keys 52 and their respective mass body units operate similarly to the white keys 51 and their respective mass body units.
The functions of each of the mass body units 55 are not only to give a key-depressing finger a feeling of mass but also to achieve characteristics of the entire keyboard mechanism; for example, each of the mass body units 55 provides a “feeling of stop” by striking or abutting against the lower-limit or upper-limit stopper 60 or 61.
However, the length and rigidity of the mass body units 55, shapes of the portions (such as the rear end portions and bent extension portions 59a) of the elongated members 59) abutting against the lower-limit and upper-limit stoppers 60 and 61, etc. are subjected to various limitations in order to achieve a good feeling of stop.
Particularly, in a case where it is desired to not employ a metal rod of a circular cross-sectional shape as the elongated member 59 in order to reduce the weight of the electronic keyboard instrument, there is a need to minimize decrease in the rigidity of the elongated member 59. If the elongated member 59 has a small rigidity, the elongated member 59 would give a poor response (reactive force) because the elongated member 59 itself would be greatly distorted. Thus, it is conceivable to employ a pipe of a hollow circular cross-sectional shape as the elongated member 59. To form a bendable, thin sheet metal plate into such a pipe of a hollow circular cross-sectional shape using a mandrel, the sheet metal plate is bent arcuately along its length. However, if a long pipe is to be formed, there would arise various problems, such as the one that the mandrel can not be pulled out of the pipe or may break after the bending due to a great frictional force between the mandrel and the pipe.